My son is fully bilingual in Afrikaans(sort of a Dutch Dialect) and English. When he was a toddler he would speak fluently to his grandmother in Afrikaans and would turn in the same conversation and seamlessly talk to me or his mother in English. He kept this ability right into about the middle years in Elementary School. After that this ability somehow diminished and he would throw English words into an Afrikaans co…
Three of my siblings are bi-lingual Russian/English. At first they mixed things up terribly, but after their Russian grandmother spent a year with them they were suddenly able to completely separate the two. Grandma doesn't speak a word of English, whereas both parents speak both languages, so we speculated that what they were doing was 'the easiest thing that works'.
When I speak in Norwegian I toss in English words all the time (because I can get away with it - all Norwegians seem to speak English). In French I don't leak English into it. In English, I rarely (if ever) slip Norwegian or French into it.
Where I really get into trouble is when I'm in a conversation with Norwegians, French, and English-speakers at the same time, because I end up trying to translate Norwegian to French, French to English, etc, and end up speaking the wrong language to pretty much everyone involved.